1. Field of the Invention.
The present invention relates to the field of telephone line interfaces.
2. Prior Art.
In telephone systems a typical unused telephone line is maintained at a DC voltage on the order of 50 volts. When a call is to be made, an off-hook signal is communicated to the central office by the originating telephone (or other device coupled to the phone line). The off-hook signal is merely comprised of a line current exceeding a predetermined threshold, typically on the order of 20 milliamps, generated by switching a substantial load on the line at the originating unit. The central office, sensing the increase in line current, responds with a relatively low level, moderate frequency dial tone, which the originating caller (human or electronic device) recognizes and responds to, typically by providing the appropriate touch tone signals on the line to identify the unit being called. Assuming the unit being called is not being used, the line for that unit will of course be at approximately 50 volts DC also. The central office then imposes a relatively high voltage low frequency AC ring signal on the line to the unit being called. The ring signal, which generally is substantially below the normal audio frequency range of the telephone, may have a frequency anywhere in the range of 16 to 68 Hz. Finally, when the called unit goes off hook, signaled by the switching of a substantial load across the phone lines by the answering or called unit, the central office will connect the calling and called unit for so long as the call continues. Completion of the call is signaled by either unit effectively going back on hook, as signaled by the removal of the load on the line at the unit going back on hook, at which time the central office will drop the line connection.
Though the bandwidth of a conventional telephone system may vary somewhat depending on the system, the bandwidth of the usual system is not very broad, as good intelligible voice reproduction will be achieved if the band is on the order of 500 to 3500 Hz, or even less. Generally speaking the dial tone and touch tones are well within this frequency range, and accordingly can be processed by at least some of the same circuits used for processing of transmitted or received signals. The ring signal frequency, however, may be only a small fraction of the lower end of the band pass of the system.
A common method of directly coupling to a phone line and imposing the required load on the line to provide an off-hook signal is to couple the primary of a small transformer to the line by a small relay. The transformer is characteristically designed to have a sufficiently low DC resistance to provide the required load on the line without substantial saturation, so that frequencies within the pass band of the phone system may be coupled to and from the line through the transformer. The ring signal, however, being of a particularly high voltage and low frequency, does not pass such transformers to any substantial extent. For this reason, characteristically prior art systems with an automatic answer capability, such as by way of example, data sets and the like, wherein the existence of a ring signal on a phone line has to be electronically detected, utilized ring detect circuitry coupled directly to the phone line rather than to the secondary of the coupling and off-hook load transformer. Because of this, such circuitry must be capable of operating at relatively high voltages for prolonged periods of time since, as stated before, phone lines are normally maintained at approximately 50 volts DC between calls. Beyond this however, the prior art ring detect circuitry must also withstand very high momentary voltages resulting from weather disturbances and the like, all of which, of course, require that the ring detect circuitry also provide DC isolation from the line so as to eliminate possible shock hazards. For this reason, prior art ring detect circuitry could not be provided in integrated circuit form, but instead, such circuitry was generally in discreet component form and included an optoisolator to provide the desired DC isolation. While such circuitry functions well, it is relatively expensive and large because of the component count, the size and cost of the optoisolator and the circuit board area these components require.